Semiconductor device

ABSTRACT

An object is to provide a semiconductor device using an oxide semiconductor having stable electric characteristics and high reliability. A transistor including the oxide semiconductor film in which a top surface portion of the oxide semiconductor film is provided with a metal oxide film containing a constituent similar to that of the oxide semiconductor film and functioning as a channel protective film is provided. In addition, the oxide semiconductor film used for an active layer of the transistor is an oxide semiconductor film highly purified to be electrically i-type (intrinsic) by heat treatment in which impurities such as hydrogen, moisture, a hydroxyl group, or a hydride are removed from the oxide semiconductor and oxygen which is a major constituent of the oxide semiconductor and is reduced concurrently with a step of removing impurities is supplied.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/183,755, filed Feb. 19, 2014, now allowed, which is a continuation ofU.S. application Ser. No. 13/080,046, filed Apr. 5, 2011, now U.S. Pat.No. 8,659,013, which claims the benefit of a foreign priorityapplication filed in Japan as Serial No. 2010-090539 on Apr. 9, 2010,all of which are incorporated by reference.

TECHNICAL FIELD

The present invention relates to a semiconductor device and amanufacturing method thereof.

In this specification, a semiconductor device means a general devicewhich can function by utilizing semiconductor characteristics, and anelectro-optic device, a semiconductor circuit, and an electronic deviceare all semiconductor devices.

BACKGROUND ART

A technique by which transistors are formed using semiconductor thinfilms formed over a substrate having an insulating surface has beenattracting attention. Such transistors are applied to a wide range ofelectronic devices such as an integrated circuit (IC) and an imagedisplay device (display device). As semiconductor thin films applicableto the transistors, silicon-based semiconductor materials have beenwidely used, but oxide semiconductors have been attracting attention asalternative materials.

For example, disclosed is a transistor whose active layer is formedusing an amorphous oxide containing indium (In), gallium (Ga), and zinc(Zn) and having an electron carrier concentration of lower than 10¹⁸/cm³(see Patent Document 1).

A transistor including an oxide semiconductor is known to have a problemof low reliability because of high possibility of change in electriccharacteristics, although the transistor including an oxidesemiconductor can be operated at higher speed than a transistorincluding amorphous silicon and can be manufactured more easily than atransistor including polycrystalline silicon. For example, a BT testunder light is performed, so that the threshold voltage of thetransistor fluctuates. On the other hand, Patent Documents 2 and 3 eachdisclose a technique of preventing charge trapping at the interface ofan oxide semiconductor layer with the use of an interfacial stabilitylayer, which is provided on at least one of the top surface and thebottom surface of the oxide semiconductor layer, in order to suppressthe shift of the threshold voltage of the transistor including an oxidesemiconductor.

REFERENCE

[Patent Document 1] Japanese Published Patent Application No.2006-165528

[Patent Document 2] Japanese Published Patent Application No.2010-016347

[Patent Document 3] Japanese Published Patent Application No.2010-016348

Disclosure of Invention

The transistor disclosed in Patent Document 2 or 3, however, includes asthe interfacial stability layer a layer having same characteristics asthose of a gate insulating layer and a protective layer, so that thestate of the interface with an active layer cannot be kept favorably.This is why it is difficult to suppress charge trapping at the interfacebetween the active layer and the interfacial stability layer. Inparticular, in the case where the interfacial stability layer and theactive layer have equivalent band gaps, charge is likely to be stored.

Thus, a transistor including an oxide semiconductor cannot yet be saidto have sufficiently high reliability.

In view of the above problems, an object is to stabilize electriccharacteristics of a semiconductor device including an oxidesemiconductor to increase reliability.

One embodiment of the disclosed invention is based on the followingtechnical idea: a metal oxide film functioning as a channel protectivefilm of the oxide semiconductor film is provided between and in contactwith the oxide semiconductor film and contains a constituent similar tothat of the oxide semiconductor film. In other words, one embodiment ofthe disclosed invention includes a layered structure of a metal oxidefilm and an oxide semiconductor film. Here, containing “a constituentsimilar to that of the oxide semiconductor film” means containing one ormore of metal elements selected from constituents of the oxidesemiconductor film.

Such a layered structure makes it possible to sufficiently suppresstrapping of charge or the like, which is generated due to the operationof a semiconductor device, or the like, at the interface of theinsulating film and the oxide semiconductor film. This advantageouseffect is brought by the following mechanism: the metal oxide filmcontaining a material compatible with the oxide semiconductor film isprovided in contact with the oxide semiconductor film, wherebysuppressed is trapping of charge or the like, which can be generated dueto the operation of a semiconductor device, at the interface between theoxide semiconductor film and the metal oxide film.

Since trapping of charge at the interface of the oxide semiconductorfilm can be suppressed, operation malfunctions of the semiconductordevice can be reduced to increase reliability of the semiconductordevice.

Further, a structure is preferable in which an insulating filmcontaining a constituent different from the constituents contained inthe metal oxide film and the oxide semiconductor film is provided overand in contact with the metal oxide film in such a layered structure.That is, one embodiment of the disclosed invention includes a layeredstructure in which an oxide semiconductor film, a metal oxide film, andan insulating film are layered.

In such a manner, an insulating film containing a material with which acharge trapping center can be formed at an interface is provided incontact with the metal oxide film, whereby charge can be trappedpreferentially at the interface between the metal oxide film and theinsulating film compared to the interface between the oxidesemiconductor film and the metal oxide film. That is to say, when theinsulating film is provided in contact with the metal oxide film, chargeis trapped preferentially at the interface of the metal oxide film andthe insulating film, so that trapping of charge at the interface betweenthe oxide semiconductor film and the metal oxide film can be suppressedmore effectively.

Since trapping of charge at the interface of the oxide semiconductorfilm can be suppressed and a charge trapping center can be kept awayfrom the oxide semiconductor film, operation malfunctions of thesemiconductor device can be reduced to increase reliability of thesemiconductor device.

In the above mechanism, the metal oxide film desirably has an enoughthickness. This is because the influence of charge trapped at theinterface between the metal oxide film and the insulating film may begreat when the metal oxide film is thin. For example, the metal oxidefilm is preferably thicker than the oxide semiconductor film.

The metal oxide film having an insulating property is formed so as notto hinder connection between the oxide semiconductor film and source anddrain electrodes, so that resistance can be prevented from beingincreased as compared to the case where a metal oxide film is providedbetween an oxide semiconductor film and a source electrode or a drainelectrode. Thus, it is possible to suppress deterioration of electriccharacteristics of the transistor.

When the composition of an oxide semiconductor differs from thestoichiometric composition because of an excess or a deficiency ofoxygen, or hydrogen or moisture which serves as an electron donor entersthe oxide semiconductor in a thin film formation process, theconductivity of the oxide semiconductor is changed. Such a phenomenon isa factor of a change in electric characteristics of the transistorincluding such an oxide semiconductor. Therefore, an oxide semiconductorfilm is highly purified to be electrically i-type (intrinsic) byremoving impurities such as hydrogen, moisture, a hydroxyl group, or ahydride (also referred to as a hydrogen compound) from the oxidesemiconductor and supplying oxygen which is a major constituent of theoxide semiconductor and is reduced concurrently with a step of removingimpurities.

An i-type (intrinsic) oxide semiconductor is an oxide semiconductorhighly purified to be i-type (intrinsic) or substantially i-type(intrinsic) by removing hydrogen, which is an n-type impurity, from theoxide semiconductor so that impurities that are not main components ofthe oxide semiconductor are contained as little as possible.

Note that in the process of making an oxide semiconductor film an i-typeoxide semiconductor film, the metal oxide film containing a constituentsimilar to that of the oxide semiconductor film can also be made ani-type film at the same time. According td one embodiment of thedisclosed invention, metal oxide films provided on a top surface and abottom surface of an oxide semiconductor film are desirably madeelectrically intrinsic by sufficiently reducing impurities such asmoisture and hydrogen.

The electric characteristics of a transistor including a highly-purifiedoxide semiconductor film, such as the threshold voltage and an off-statecurrent, have almost no temperature dependence. Further, transistorcharacteristics are less likely to change due to light deterioration.

One embodiment of the present invention will be described in detailbelow.

One embodiment of the disclosed invention is a semiconductor deviceincluding a gate electrode; a gate insulating film covering the gateelectrode; an oxide semiconductor film provided over the gate insulatingfilm and in a region overlapping with the gate electrode; a metal oxidefilm over and in contact with the oxide semiconductor film; and a sourceelectrode and a drain electrode over the metal oxide film and in contactwith parts of the oxide semiconductor film. The metal oxide filmcontains an oxide containing one or more of metal elements selected fromconstituent elements of the oxide semiconductor film.

Note that, in the above, the semiconductor device may include aninsulating film provided over and in contact with the metal oxide filmand covering the source electrode and the drain electrode. Further, inthe above, a conductive film may be provided over the insulating film.

In the above, the width in a channel length direction of the metal oxidefilm may be smaller than that of the oxide semiconductor film, and thesource electrode and the drain electrode may be in contact with parts ofa top surface of the oxide semiconductor film. In the above, side edgesin a channel length direction of the oxide semiconductor film may bealigned with those of the metal oxide film. In the above, the metaloxide film may have a structure in which at least the oxidesemiconductor film is covered with the metal oxide film, openings areprovided so as to expose parts of the oxide semiconductor film, and thesource electrode and the drain electrode are in contact with the oxidesemiconductor film in the openings. In the above, a second metal oxidefilm may be provided over and in contact with the gate insulating filmand in contact with a bottom surface of the oxide semiconductor film. Inthe above, the metal oxide film preferably functions as a channelprotective film.

In the above, a protective insulating film may be provided over and incontact with the metal oxide film and in contact with parts of topsurfaces of the source electrode and the drain electrode. Note that theprotective insulating film functions as a film protecting a channelformation region of the oxide semiconductor film when the sourceelectrode and the drain electrode are etched.

In the above, the metal oxide film has preferably a larger energy gapthan the oxide semiconductor film. Energy at the bottom of theconduction band of the metal oxide film is preferably higher than thatof the oxide semiconductor film.

In the above, the metal oxide film may contain a gallium oxide.

In the above, the gate insulating film may contain a silicon oxide or ahafnium oxide.

In the above, the channel length L of the transistor, which depends onthe width in the channel length direction of the metal oxide filmfunctioning as a channel protective film can be greater than or equal to10 nm and less than or equal to 10 μm, for example, greater than orequal to 0.1 μm and less than or equal to 0.5 μm. The channel length Lmay be 1 μm or more. Further, the channel width W may be 10 nm or more.

According to one embodiment of the present invention, a transistorhaving stable electric characteristics can be manufactured.

According to one embodiment of the present invention, a semiconductordevice including a highly reliable transistor having favorable electriccharacteristics can be manufactured.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are a plan view and cross-sectional views illustrating anembodiment of a semiconductor device;

FIG. 2 is a band diagram of a transistor including an oxidesemiconductor film and a metal oxide film;

FIGS. 3A to 3C are a plan view and cross-sectional views illustrating anembodiment of a semiconductor device;

FIGS. 4A to 4H are cross-sectional views each illustrating an embodimentof a semiconductor device;

FIGS. 5A to 5E are cross-sectional views illustrating an example of amanufacturing process of the semiconductor device in FIGS. 1A to 1C;

FIGS. 6A to 6C are views each illustrating an embodiment of asemiconductor device;

FIG. 7 is a cross-sectional view illustrating an embodiment of asemiconductor device;

FIG. 8 is a cross-sectional view illustrating an embodiment of asemiconductor device;

FIG. 9 is a cross-sectional view illustrating an embodiment of asemiconductor device; and

FIGS. 10A to 10F are views illustrating electronic devices.

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. Note that thepresent invention is not limited to the description below, and it iseasily understood by those skilled in the art that modes and detailsdisclosed herein can be modified in various ways without departing fromthe spirit and the scope of the present invention. Thus, the presentinvention is not construed as being limited to description of theembodiments.

Note that the ordinal numbers such as “first” and “second” in thisspecification are used for convenience and do not denote the order ofsteps or the stacking order of layers. In addition, the ordinal numbersin this specification do not denote particular names which specify thepresent invention.

EMBODIMENT 1

In this embodiment, one embodiment of a semiconductor device and amanufacturing method thereof will be described with reference to FIGS.1A to 1C, FIG. 2, FIGS. 3A to 3C, FIGS. 4A to 4H, and FIGS. 5A to 5E.

<Structural Example of Semiconductor Device>

In FIGS. 1A to 1C, a cross-sectional view and a plan view of achannel-protective transistor (also referred to as a channel-stoptransistor), which is one of bottom-gate transistors, are illustrated asan example of a semiconductor device. FIG. 1A is a plan view and FIGS.1B and 1C are cross-sectional views taken along line A-B and line C-D inFIG. 1A, respectively. In FIG. 1A, some of components of a transistor310 (for example, a gate insulating film 402) are omitted for brevity.

The transistor 310 in FIG. 1A includes, over a substrate 400 having aninsulating surface, a gate electrode 401, a gate insulating film 402covering the gate electrode 401, an oxide semiconductor film 403provided over the gate insulating film 402 and in a region overlappingwith the gate electrode 401, a metal oxide film 407 provided over and incontact with the oxide semiconductor film 403, and a source electrode405 a and a drain electrode 405 b provided over the metal oxide film 407and in contact with parts of the oxide semiconductor film 403. The metaloxide film 407 functions as a channel protective film in the transistor310 in FIG. 1A. Further, the transistor 310 preferably includes aninsulating film 409 covering the source electrode 405 a and the drainelectrode 405 b and over and in contact with the metal oxide film 407.

Here, it is desirable to use an oxide containing a constituent similarto that of the oxide semiconductor film 403 for the metal oxide film407. Specifically, the metal oxide film 407 is preferably a filmcontaining an oxide containing one or more of metal elements selectedfrom constituent elements of the oxide semiconductor film. This isbecause such a material is compatible with the oxide semiconductor film403 and thus, when it is used for the metal oxide film 407, the state ofthe interface with the oxide semiconductor film can be kept favorably.That is to say, the use of the above material for the metal oxide film407 makes it possible to suppress trapping of charge at the interfacebetween the oxide semiconductor film and the metal oxide film in contactwith the oxide semiconductor film (here, the interface between the metaloxide film 407 and the oxide semiconductor film 403).

The metal oxide film 407 needs to have a larger energy gap than theoxide semiconductor film 403 because the oxide semiconductor film 403 isused as an active layer. In addition, it is necessary to form at leastan energy barrier between the metal oxide film 407 and the oxidesemiconductor film 403, in which carriers do not flow from the oxidesemiconductor film 403 at room temperature (20° C.). For example, theenergy difference between the bottom of the conduction band of the metaloxide film 407 and the bottom of the conduction band of the oxidesemiconductor film 403 or the energy difference between the top of thevalence band of the oxide semiconductor film 403 and the top of thevalence band of the metal oxide film 407 is desirably 0.5 eV or more,more desirably 0.7 eV or more. In addition, the energy differencetherebetween is desirably 1.5 eV or less.

Specifically, for example, when an In—Ga—Zn—O-based material is used forthe oxide semiconductor film 403, the metal oxide film 407 may be formedusing a material containing gallium oxide, or the like. When the galliumoxide is in contact with the In—Ga—Zn—O-based material, the energybarrier is about 0.8 eV on the conduction band side and about 0.9 eV onthe valence band side.

Note that a gallium oxide is also referred to as GaO_(x) and the valueof x is preferably set so that the oxygen amount exceeds thestoichiometric proportion. For example, the value of x is preferably setto larger than or equal to 1.4 and smaller than or equal to 2.0, furtherpreferably larger than or equal to 1.5 and smaller than or equal to 1.8.When a gallium oxide is used as the metal oxide film 407, it isdesirable that a gallium oxide film is a film from which impurities suchas hydrogen and water are sufficiently reduced. Note that a galliumoxide film may contain an impurity element other than hydrogen, e.g., anelement belonging to Group 3 such as yttrium, an element belonging toGroup 4 such as hafnium, an element belonging to Group 13 such asaluminum, an element belonging to Group 14 such as silicon, or nitrogenso that the energy gap of the gallium oxide is increased to improve theinsulating property. The energy gap of a gallium oxide film which doesnot contain an impurity is 4.9 eV; however, when the gallium oxide filmcontains any of the above impurities at about greater than 0 atomic %and less than or equal to 20 atomic %, the energy gap can be increasedto about 6 eV.

In order to reduce charge sources and charge trapping centers, it isdesirable to sufficiently reduce impurities such as hydrogen and waterin the metal oxide film. This idea is similar to the idea of reductionof impurities in an oxide semiconductor film.

The metal oxide film 407 functioning as a channel protective film isprovided in a region overlapping with a channel formation region of theoxide semiconductor film 403; thus, damage to the channel formationregion by etching of the source electrode 405 a and the drain electrode405 b (e.g., damage due to plasma or etchant in etching) can beprevented. Thus, a semiconductor device including an oxide semiconductorwith stable electric characteristics can be provided.

The source electrode 405 a and the drain electrode 405 b are in contactwith parts of a top surface of the oxide semiconductor film 403, whenthe width in the channel length direction of the metal oxide film 407 issmaller than that of the oxide semiconductor film 403 as in FIG. 1B.That is, the width in the channel length direction of the metal oxidefilm 407 is made small, so that the channel length of the transistor 310is made small; therefore, an increase in operation speed and a reductionin power consumption of the transistor can be achieved.

The transistor 310 has a structure in which a contact area between thesource and drain electrodes 405 a and 405 b and the oxide semiconductorfilm 403 is reduced compared to a so-called channel-etched transistor inwhich the metal oxide film 407 is not provided; thus, a region aroundthe interface between the source and drain electrodes 405 a and 405 band the oxide semiconductor film 403 becomes a highly resistant region.Thus, the concentration of the electric field in the transistor 310 canbe alleviated, whereby a short-channel effect can be suppressed evenwhen the transistor 310 is downsized.

When the insulating film 409 is provided over and in contact with themetal oxide film 407, it is desirable to use a material with which acharge trapping center can be formed at the interface with the metaloxide film 407 when the material is in contact with the metal oxide film407, for the insulating film 409. By using such a material for theinsulating film 409, charge is preferentially trapped at the interfacebetween the insulating film 409 and the metal oxide film 407, so that itis possible to effectively suppress trapping of charge at the interfacebetween the metal oxide film 407 and the oxide semiconductor film 403.Note that when many charge trapping centers are formed at the interfacebetween the insulating film 409 and the metal oxide film 407, transistorcharacteristics might possibly get worse; thus, it is favorable thatcharge trapping centers be slightly more likely to be formed at theinterface between the insulating film 409 and the metal oxide film 407than at the interface between the oxide semiconductor film 403 and themetal oxide film 407.

Specifically, the insulating film 409 may be formed to have asingle-layer or layered structure using any of a silicon oxide, asilicon nitride, an aluminum oxide, an aluminum nitride, a mixedmaterial of any of them, and the like. For example, when a materialcontaining a gallium oxide is used for the metal oxide film 407, asilicon oxide, a silicon nitride, or the like is preferably used for theinsulating film 409. In addition, the energy gap of the insulating film409 is desirably larger than that of the metal oxide film 407 becausethe insulating film 409 is in contact with the metal oxide film 407.

Note that it is not necessary to limit the material of the insulatingfilm 409 to the above as long as a charge trapping center can be formedat the interface between the insulating film 409 and the metal oxidefilm 407. Further, treatment through which a charge trapping center isformed may be performed on the interface between the insulating film 409and the metal oxide film 407. As such treatment, plasma treatment andtreatment for adding an element (ion implantation or the like) aregiven, for example.

Note that the metal oxide film 407 is patterned into an island shape inthe transistor 310; however, the metal oxide film 407 is not necessarilypatterned into an island shape. In addition, side edges in a channellength direction of the oxide semiconductor film 403 may be aligned withthose of the metal oxide film 407. When the insulating film 409 isformed, a second gate electrode may be further formed over the oxidesemiconductor film 403. In this case, the transistor 310 may be atop-gate transistor in which the gate electrode 401 is not provided. Asecond metal oxide film may be further provided over and in contact withthe gate insulating film 402. The oxide semiconductor film 403 may bepatterned so that the width in the channel length direction of the oxidesemiconductor film 403 is smaller than that of the gate electrode 401.An insulating film may be further provided over the transistor 310.Further, openings may be formed in the gate insulating film 402, themetal oxide film 407, the insulating film 409, and the like in orderthat the source electrode 405 a and the drain electrode 405 b may beelectrically connected to a wiring. Note that it is not always necessarybut desirable to process the oxide semiconductor film 403 into an islandshape.

FIG. 2 is an energy band diagram (schematic diagram) of the transistor310, that is, an energy band diagram of the structure where theinsulating film, the oxide semiconductor film, the metal oxide film, andthe insulating film are bonded to each other from the gate electrode GEside, and E_(F) denotes the Fermi level of the oxide semiconductor film.FIG. 2 shows the case where a silicon oxide (SiO_(x)) (with a band gapEg of 8 eV to 9 eV), a gallium oxide (GaO_(x)) (with a band gap Eg of4.9 eV), and an In—Ga—Zn—O-based non-single-crystal film (with a bandgap Eg of 3.15 eV) are used as the insulating film, the metal oxidefilm, and the oxide semiconductor (OS) film, respectively, on theassumption of the ideal state where the insulating films, the metaloxide films, and the oxide semiconductor film are all intrinsic. Notethat the energy difference between the vacuum level and the bottom ofthe conduction band of the silicon oxide is 0.95 eV, the energydifference between the vacuum level and the bottom of the conductionband of the gallium oxide is 3.5 eV, and the energy difference betweenthe vacuum level and the bottom of the conduction band of theIn—Ga—Zn—O-based non-single-crystal film is 4.3 eV.

As shown in FIG. 2, on the gate electrode side (the channel side) of theoxide semiconductor film, energy barriers of about 3.35 eV and about 2.5eV exist at the interface between the oxide semiconductor film and theinsulating film. On the side opposite to the gate electrode (the backchannel side) of the oxide semiconductor film, similarly, energybarriers of about 0.8 eV and about 0.95 eV exist at the interfacebetween the oxide semiconductor film and the metal oxide film. When suchenergy barriers exist at the interface between the oxide semiconductorfilm and the insulating film and at the interface between the oxidesemiconductor film and the metal oxide film, transport of carriers atthe interfaces can be prevented; thus, the carriers do not travel fromthe oxide semiconductor film to the insulating film and from the oxidesemiconductor film to the metal oxide film, and the carriers travelthrough the oxide semiconductor film. In other words, the oxidesemiconductor film is provided so as to be sandwiched between materialswhose band gaps are each larger than that of the oxide semiconductorfilm (here, the metal oxide film and the insulating film), whereby thecarriers travel through the oxide semiconductor film.

FIGS. 3A to 3C and FIGS. 4A to 4H illustrate structural examples oftransistors having different structures from that in FIGS. 1A to 1C.

FIGS. 3A to 3C illustrate cross-sectional views and a plan view of atransistor having a structure in which the metal oxide film 407 coversthe oxide semiconductor film 403. Here, FIG. 3A is a plan view, FIGS. 3Band 3C are cross-sectional views taken along line A-B and line C-D inFIG. 3A, respectively. Note that in FIG. 3A, some of components of atransistor 320 (e.g., the gate insulating film 402) are omitted forbrevity.

The transistor 320 in FIG. 3A is the same as the transistor 310 in FIGS.1A to 1C in that it includes, over the substrate 400, the gate electrode401, the gate insulating film 402, the oxide semiconductor film 403, themetal oxide film 407, the source electrode 405 a, the drain electrode405 b, and the insulating film 409. The difference between thetransistor 320 in FIG. 3A and the transistor 310 in FIGS. 1A to 1C isthat the metal oxide film 407 covers the oxide semiconductor film 403.Here, in the transistor 320, the source and drain electrodes 405 a and405 b are in contact with the oxide semiconductor film 403 in openingsformed in the metal oxide film 407 so as to expose parts of the oxidesemiconductor film 403. The other components are the same as those ofthe transistor 310 in FIGS. 1A to 1C; thus, the description on FIGS. 1Ato 1C can be referred to for the details.

With such a structure, the transistor 320 has a structure in which acontact area between the source and drain electrode 405 a and 405 b andthe oxide semiconductor film 403 is reduced compared to the transistor310 in FIGS. 1A to 1C; thus, a region around the interface between thesource and drain electrodes 405 a and 405 b and the oxide semiconductorfilm 403 becomes a more highly resistant region. Thus, the concentrationof the electric field in the transistor 310 can be further alleviated,whereby a short-channel effect can be suppressed more effectively evenwhen the transistor 310 is downsized.

Transistors 330 and 340 in FIGS. 4A and 4B are different from thetransistors 310 and 320, respectively, in that a conductive film 410 isprovided over the insulating film 409 and in a region overlapping with achannel formation region of the oxide semiconductor film 403. Theconductive film 410 may be formed using a material and a method whichare similar to those of the gate electrode 401. The other components arethe same as those of the transistors 310 and 320. Note that transistors350 and 360 in FIGS. 4C and 4D are different from the transistors 330and 340 in that the gate electrode 401 and the gate insulating film 402are not provided and the transistors 350 and 360 are top-gatetransistors.

Transistors 370 and 380 in FIGS. 4E and 4F are different from thetransistors 310 and 320 in that the metal oxide film 404 is furtherprovided over and in contact with the gate insulating film 402. Themetal oxide film 404 may be formed using a material and a method whichare similar to those of the metal oxide film 407. The gate insulatingfilm 402 is preferably formed using a material and a method which aresimilar to those of the insulating film 409. The other components arethe same as those of the transistors 310 and 320.

With such a structure, trapping of charge can be suppressed even at abottom surface portion of the oxide semiconductor film 403, that is, atthe interface between the oxide semiconductor film 403 and the metaloxide film 404. By using such a material with which a charge trappingcenter can be formed at the interface with the metal oxide film 404 whenthe material is in contact with the metal oxide film 404, for the gateinsulating film 402, charge is trapped preferentially at the interfacebetween the gate insulating film 402 and the metal oxide film 404, sothat trapping of charge at the interface between the metal oxide film404 and the oxide semiconductor film 403 can be suppressed moreeffectively.

The transistor 390 in FIG. 4G is different from the transistor 310 inthat the oxide semiconductor film 403 is patterned so that the width inthe channel length direction of the oxide semiconductor film 403 issmaller than that of the gate electrode 401. The other components arethe same as those of the transistor 310. With such a structure, theoxide semiconductor film 403 can have a flat shape, so that carrierscattering can be prevented and interface levels can be reduced at theinterface between the oxide semiconductor film 403 and the gateinsulating film 402.

A transistor 500 in FIG. 4H is different from the transistor 310 in thata protective insulating film 419 is further provided over and in contactwith the metal oxide film 407. That is, parts of a top surface of theprotective insulating film 419 are in contact with the source electrode405 a and the drain electrode 405 b, and the protective insulating film419 functions as a channel protective film together with the metal oxidefilm 407. The protective insulating film 419 may be formed using amaterial and a method which are similar to those of the insulating film409. The other components are the same as those of the transistors 310.

With such a structure, even if the insulating film 409 is not provided,the protective insulating film 419 can be provided over and in contactwith the metal oxide film 407 using such a material with which a chargetrapping center can be formed at the interface with the metal oxide film407 when the material is in contact with the metal oxide film 407. Thus,even if the insulating film 409 is not provided, charge is trappedpreferentially at the interface between the protective insulating film419 and the metal oxide film 407 compared to at the interface betweenthe oxide semiconductor film 403 and the metal oxide film 407, so thattrapping of charge at the interface between the metal oxide film 407 andthe oxide semiconductor film 403 can be suppressed more effectively.

Note that the structures of the transistors can be combined with eachother as appropriate.

<Example of Manufacturing Process of Transistor>

Hereinafter, examples of manufacturing processes of the transistorsillustrated in FIGS. 1A to 1C, FIGS. 3A to 3C, and FIGS. 4A to 4H willbe described with reference to FIGS. 5A to 5E.

<Manufacturing Process of Transistor 310>

An example of a manufacturing process of the transistor 310 illustratedin FIGS. 1A to 1C will be described with reference to FIGS. 5A to 5E.

First, a conductive film is formed over the substrate 400 having aninsulating surface, and then, the gate electrode 401 is formed in afirst photolithography step (see FIG. 5A). Note that a resist mask maybe formed by an inkjet method. Formation of the resist mask by an inkjetmethod needs no photomask; thus, manufacturing cost can be reduced.

Although there is no particular limitation on a substrate which can beused as the substrate 400 having an insulating surface, it is necessarythat the substrate have at least enough heat resistance to heattreatment to be performed later. For example, a glass substrate, aceramic substrate, a quartz substrate, or a sapphire substrate can beused. Alternatively, a single crystal semiconductor substrate or apolycrystalline semiconductor substrate of silicon, silicon carbide, orthe like; a compound semiconductor substrate of silicon germanium or thelike; an SOI substrate, or the like can be used as long as the substratehas an insulating surface. In addition, semiconductor elements may beprovided over these substrates.

A flexible substrate may be used as the substrate 400. When a flexiblesubstrate is used, a transistor including the oxide semiconductor film403 may be directly formed over the flexible substrate. Alternatively, atransistor including the oxide semiconductor film 403 may be formed overa manufacturing substrate, and then, the transistor may be separated andtransferred to a flexible substrate. Note that in order to separate andtransfer a transistor including the oxide semiconductor film 403 fromthe manufacturing substrate to the flexible substrate, a separationlayer is preferably provided between the manufacturing substrate and thetransistor including the oxide semiconductor film 403.

An insulating film functioning as a base film may be provided betweenthe substrate 400 and the gate electrode 401. The base film may have afunction of preventing diffusion of an impurity element from thesubstrate 400, and can be formed with a single-layer or layeredstructure using one or more of a silicon nitride film, a silicon oxidefilm, a silicon nitride oxide film, and a silicon oxynitride film.

The gate electrode 401 can be formed with a single layer or a stacklayer using a metal material such as molybdenum, titanium, tantalum,tungsten, aluminum, copper, neodymium, or scandium, or an alloy materialwhich includes any of these materials as a main component.

Next, the gate insulating film 402 is formed over the gate electrode 401(see FIG. 5A).

Specifically, the gate insulating film 402 may be formed to have asingle-layer or layered structure using any of a silicon oxide film, asilicon nitride film, a silicon oxynitride film, a silicon nitride oxidefilm, an aluminum oxide film, an aluminum nitride film, an aluminumoxynitride film, an aluminum nitride oxide film, a hafnium oxide film,and a gallium oxide film.

There is no particular limitation on the method for forming the gateinsulating film 402, and for example, the gate insulating film 402 maybe formed by a deposition method such as a plasma CVD method or asputtering method.

Note that after the gate insulating film 402 is formed, the metal oxidefilm 404 is further formed over the gate insulating film 402, so thatthe transistor 370 in FIG. 4E and the transistor 380 in FIG. 4F can beformed. The metal oxide film 404 can be formed using a material and amethod which are similar to those of the metal oxide film 407 to bedescribed later.

Next, the oxide semiconductor film 403 having a thickness of greaterthan or equal to 3 nm and less than or equal to 30 nm is formed over thegate insulating film 402 by a sputtering method (see FIG. 5A). The abovethickness is preferable because the transistor might possibly benormally on when the oxide semiconductor film 403 is too thick (e.g.,the thickness is 50 nm or more). Note that the gate insulating film 402and the oxide semiconductor film 403 is preferably formed successivelywithout being exposed to the air.

Note that before the oxide semiconductor film 403 is formed by asputtering method, powdery substances (also referred to as particles ordust) which are attached on a surface of the gate insulating film 402are preferably removed by reverse sputtering in which an argon gas isintroduced and plasma is generated. The reverse sputtering refers to amethod in which a voltage is applied to a substrate side to generateplasma in the vicinity of the substrate to modify a surface. Note thatinstead of argon, a gas of nitrogen, helium, oxygen or the like may beused.

As an oxide semiconductor used for the oxide semiconductor film 403, anyof the following oxide semiconductors can be used: anIn—Sn—Ga—Zn—O-based oxide semiconductor which is an oxide of four metalelements; an In—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-basedoxide semiconductor, an In—Al—Zn—O-based oxide semiconductor, aSn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxidesemiconductor, and a Sn—Al—Zn—O-based oxide semiconductor which areoxides of three metal elements; an In—Zn—O-based oxide semiconductor, aSn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor,a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxidesemiconductor, and an In—Mg—O-based oxide semiconductor which are oxidesof two metal elements; and an In—O-based oxide semiconductor, aSn—O-based oxide semiconductor, and a Zn—O-based oxide semiconductorwhich are oxides of one metal element. Further, SiO₂ may be contained inthe above oxide semiconductor. In this specification, for example, anIn—Ga—Zn—O-based oxide semiconductor means an oxide film containingindium (In), gallium (Ga), and zinc (Zn), and there is no particularlimitation on the composition ratio. The In—Ga—Zn—O-based oxidesemiconductor may contain an element other than In, Ga, and Zn.

As the oxide semiconductor film 403, a thin film formed using a materialexpressed by a chemical formula of InMO₃(ZnO)_(m) (m>0) can be used.Here, M represents one or more metal elements selected from Ga, Al, Mn,and Co. For example, M can be Ga, Ga and Al, Ga and Mn, Ga and Co, orthe like.

In this embodiment, the oxide semiconductor film 403 is formed by asputtering method using an In—Ga—Zn—O-based oxide semiconductor targetfor film formation. Alternatively, the oxide semiconductor film 403 canbe formed by a sputtering method under a rare gas (typically, argon)atmosphere, an oxygen atmosphere, or a mixed atmosphere of a rare gasand oxygen.

As a target for forming an In—Ga—Zn—O-based oxide semiconductor film asthe oxide semiconductor film 403 by a sputtering method, for example, anoxide semiconductor film formation target with the following compositionratio may be used: the composition ratio of In₂O₃:Ga₂O₃:ZnO is 1:1:1[molar ratio]. Note that it is not necessary to limit the material andthe composition ratio of the target to the above. For example, an oxidesemiconductor film formation target with the following composition ratiomay alternatively be used: the composition ratio of In₂O₃:Ga₂O₃:ZnO is1:1:2 [molar ratio].

When an In—Zn—O-based material is used for the oxide semiconductor, atarget with the following composition ratio is used: the compositionratio of In:Zn is 50:1 to 1:2 in an atomic ratio (In₂O₃:ZnO=25:1 to 1:4in a molar ratio), preferably 20:1 to 1:1 in an atomic ratio(In₂O₃:ZnO=10:1 to 1:2 in a molar ratio), further preferably 15:1 to1.5:1 in an atomic ratio (In₂O₃:ZnO=15:2 to 3:4 in a molar ratio). Forexample, a target used for the formation of an In—Zn—O-based oxidesemiconductor has the following atomic ratio: the atomic ratio ofIn:Zn:O is X:Y:Z, where Z>1.5X+Y.

In addition, the filling factor of the oxide semiconductor target forfilm formation is greater than or equal to 90% and less than or equal to100%, preferably greater than or equal to 95% and less than or equal to99.9%. With the use of the oxide semiconductor target for film formationwith a high filling factor, the oxide semiconductor film 403 can beformed to be dense.

A high-purity gas in which impurities such as hydrogen, water, hydroxyl,and hydride are removed is preferably used as a sputtering gas used information of the oxide semiconductor film 403.

For the formation of the oxide semiconductor film 403, a substrate isheld in a deposition chamber kept at reduced pressure and the substratetemperature is preferably set at higher than or equal to 100° C. andlower than or equal to 600° C., preferably higher than or equal to 200°C. and lower than or equal to 400° C. The film formation is performedwhile the substrate 400 is heated, the concentration of impuritiescontained in the oxide semiconductor film 403 can be reduced. Moreover,damage to the oxide semiconductor film 403 due to sputtering is reduced.Then, a sputtering gas from which hydrogen and moisture are removed isintroduced into the deposition chamber from which residual moisture isbeing removed, and the oxide semiconductor film 403 is formed over thesubstrate 400 with the use of the target. In order to remove residualmoisture in the deposition chamber, an entrapment vacuum pump such as acryopump, an ion pump, or a titanium sublimation pump is preferablyused. As an evacuation unit, a turbo pump to which a cold trap is addedmay be used. In the deposition chamber which is evacuated with thecryopump, for example, a hydrogen atom, a compound containing a hydrogenatom, such as water (H₂O), (further preferably, also a compoundcontaining a carbon atom), and the like are removed, whereby theconcentration of impurities in the oxide semiconductor film 403 formedin the deposition chamber can be reduced.

One example of the deposition condition is as follows: the distancebetween the substrate and the target is 100 mm, the pressure is 0.6 Pa,the direct-current (DC) power source is 0.5 kW, and the atmosphere is anoxygen atmosphere (the rate of the oxygen flow is 100%). Note that apulsed direct current power source is preferably used because powderysubstances (also referred to as particles or dust) generated in filmformation can be reduced and thickness distribution can be small.

After that, heat treatment (first heat treatment) is preferablyperformed on the oxide semiconductor film 403. Excessive hydrogen(including water and a hydroxyl group) in the oxide semiconductor film403 is removed by the first heat treatment and a structure of the oxidesemiconductor film 403 is improved, so that defect levels in energy gapof the oxide semiconductor film 403 can be reduced. The first heattreatment is performed at a temperature higher than or equal to 250° C.and lower than or equal to 700° C., preferably higher than or equal to450° C. and lower than or equal to 600° C. The temperature of the firstheat treatment is preferably lower than the strain point of thesubstrate.

The heat treatment can be performed in such a way that, for example, anobject to be heated is introduced into an electric furnace in which aresistance heating element or the like is used and heated at 450° C. ina nitrogen atmosphere for an hour. During the heat treatment, the oxidesemiconductor film 403 is not exposed to the air, in order to prevententry of water and hydrogen.

The heat treatment apparatus is not limited to an electric furnace; theheat treatment apparatus can be an apparatus that heats an object to beheated using thermal radiation or thermal conduction from a medium suchas a heated gas. For example, an RTA (rapid thermal annealing) apparatussuch as a GRTA (gas rapid thermal annealing) apparatus or an LRTA (lamprapid thermal annealing) apparatus can be used. An LRTA apparatus is anapparatus for heating an object using radiation of light (anelectromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high-pressuresodium lamp, or a high-pressure mercury lamp. A GRTA apparatus is anapparatus for heat treatment using a high-temperature gas. As the gas,an inert gas which does not react with an object in heat treatment, suchas nitrogen or a rare gas like argon, is used.

For example, as the first heat treatment, GRTA treatment may beperformed in the following manner. The object is put in an inert gasatmosphere that has been heated, heated for several minutes, and thentaken out of the inert gas atmosphere. The GRTA treatment enableshigh-temperature heat treatment in a short time. Moreover, in the GRTAtreatment, even conditions of the temperature that exceeds the uppertemperature limit of the object can be employed. Note that the gas maybe switched from the inert gas to a gas including oxygen during theprocess. This is because defect levels in the energy gap due to oxygendeficiency can be reduced by performing the first heat treatment in anatmosphere including oxygen.

Note that as the inert gas atmosphere, an atmosphere that containsnitrogen or a rare gas (e.g., helium, neon, or argon) as its maincomponent and does not contain water, hydrogen, and the like ispreferably used. For example, the purity of nitrogen or a rare gas suchas helium, neon, or argon introduced into a heat treatment apparatus isset to higher than or equal to 6N (99.9999%), preferably higher than orequal to 7N (99.99999%) (i.e., the impurity concentration is lower thanor equal to 1 ppm, preferably lower than or equal to 0.1 ppm).

In any case, when impurities are reduced by the first heat treatment toform the oxide semiconductor film 403 that is an i-type (intrinsic) orsubstantially i-type semiconductor, a transistor with extremelyexcellent characteristics can be realized.

The above heat treatment (first heat treatment) has an effect ofremoving hydrogen, water, and the like and thus can be referred to asdehydration treatment, dehydrogenation treatment, or the like. Thedehydration treatment or the dehydrogenation treatment can be performed,for example, after the oxide semiconductor film 403 is processed into anisland shape. Such dehydration treatment or dehydrogenation treatmentmay be conducted once or plural times.

Next, the oxide semiconductor film 403 is preferably processed to havean island-shaped oxide semiconductor film 403 in a secondphotolithography step (see FIG. 5A). A resist mask for forming theisland-shaped oxide semiconductor film 403 may be formed by an inkjetmethod. Formation of the resist mask by an inkjet method needs nophotomask; thus, manufacturing cost can be reduced. For the etching ofthe oxide semiconductor film 403, wet etching, dry etching, or both ofthem may be employed.

Note that the island-shaped oxide semiconductor film 403 is processed sothat the width in the channel length direction of the oxidesemiconductor film 403 is less than that of the gate electrode 401, sothat the transistor 390 in FIG. 4G can be formed.

Next, plasma treatment may be performed using a gas such as N₂O, N₂, orAr so that water adsorbed to a surface of an exposed portion of theoxide semiconductor film 403 is removed. When plasma treatment isperformed, the metal oxide film 407 is preferably formed in contact withthe oxide semiconductor film 403 without exposure to the air, followingthe plasma treatment.

Next, the metal oxide film 427 is formed to cover the oxidesemiconductor film 403 (see FIG. 5B). Note that the metal oxide film 427is processed into an island shape in a later step to be the metal oxidefilm 407.

Here, the metal oxide film 427 (the metal oxide film 407) desirablycontains a constituent similar to that of the oxide semiconductor film403 and is desirably formed using an oxide containing the mainconstituent material of the oxide semiconductor film 403. Such amaterial is compatible with the oxide semiconductor film 403; thus, whenit is used for the metal oxide film 407, the state of the interfacebetween the oxide semiconductor film and the metal oxide film 407 can bekept favorably. That is to say, the use of the above material for themetal oxide film 427 (the metal oxide film 407) makes it possible tosuppress trapping of charge at the interface between the metal oxidefilm 407 and the oxide semiconductor film 403.

The metal oxide film 407 needs to have a larger energy gap than theoxide semiconductor film 403. In addition, it is necessary to form atleast an energy barrier between the metal oxide film 407 and the oxidesemiconductor film 403, in which carriers do not flow from the oxidesemiconductor film 403 at room temperature (20° C.).

In order to reduce charge sources and charge trapping centers, it isdesirable to sufficiently reduce impurities such as hydrogen and waterin the metal oxide film 407. This idea is similar to the idea ofreduction of impurities in an oxide semiconductor film.

The metal oxide film 427 (the metal oxide film 407) is preferably formedby a method by which impurities such as water and hydrogen do not enterthe metal oxide film 427. When hydrogen is contained in the metal oxidefilm 427 (the metal oxide film 407), entry of the hydrogen into theoxide semiconductor film 403 or extraction of oxygen in the oxidesemiconductor film 403 by hydrogen may occur, thereby causing thebackchannel of the oxide semiconductor film 403 to have lower resistance(to be n-type), so that a parasitic channel may be formed. Thus, it isimportant that a deposition method in which hydrogen is not used isemployed such that the metal oxide film 427 (the metal oxide film 407)contains hydrogen as little as possible.

Therefore, the metal oxide film 427 is preferably formed by a sputteringmethod, and a high-purity gas in which impurities such as hydrogen,water, a hydroxyl group, and hydride are removed is preferably used as asputtering gas used for film formation.

The metal oxide film 427 (the metal oxide film 407) preferably has athickness large enough to keep a charge trapping center away from theoxide semiconductor film 403. Specifically, the metal oxide film 427(the metal oxide film 407) preferably has a thickness of larger than 10nm and smaller than or equal to 100 nm.

Note that after the metal oxide film 427 is formed, the protectiveinsulating film 419 is further formed over the metal oxide film 427, sothat the transistor 500 in FIG. 4H can be formed. The protectiveinsulating film 419 can be formed using a material and a method whichare similar to those of the insulating film 409 described later.Further, the protective insulating film 419 can also be patterned whenthe metal oxide film 427 is patterned into the metal oxide film 407 in alater step. The protective insulating film 419 may be patterned in adifferent step from that of the metal oxide film 427.

Next, a resist mask is formed over the metal oxide film 427 by a thirdphotolithography step, the metal oxide film 407 functioning as a channelprotective film is formed by etching, and then, the resist mask isremoved (see FIG. 5C). The resist masks for forming the metal oxide film407 may be formed by an ink-jet method. Formation of the resist mask byan inkjet method needs no photomask; thus, manufacturing cost can bereduced. For the etching of the metal oxide film 427, wet etching, dryetching, or both of them may be employed.

Light exposure at the time when the resist mask is formed in the thirdphotolithography step may be performed using ultraviolet light, KrFlaser light, or ArF laser light. The channel length L of the transistorformed later is determined, depending on the width in the channel lengthdirection of the metal oxide film 407 functioning as a channelprotective film. When light exposure is performed for a channel length Lof smaller than 25 nm, the light exposure when the resist mask is formedin the third photolithography step may be performed using extremeultraviolet light having an extremely short wavelength of severalnanometers to several tens of nanometers, for example. In the lightexposure by extreme ultraviolet light, the resolution is high and thefocus depth is large. Thus, the channel length L of the transistorformed later can be reduced, whereby the operation speed of a circuitcan be increased.

Here, patterning is performed so that the width in the channel lengthdirection of the metal oxide film 407 is smaller than that of the oxidesemiconductor film 403, whereby the source electrode 405 a and the drainelectrode 405 b are in contact with part of a top surface of the oxidesemiconductor film 403. Thus, the width in the channel length directionof the metal oxide film 407 is made small, so that the channel length ofthe transistor is made small; therefore, an increase in operation speedand a reduction in power consumption of the transistor can be achieved.

In an etching step of the metal oxide film 427, when etching selectivityratio of the metal oxide film 427 to the oxide semiconductor film 403cannot be high, part of a region of the oxide semiconductor film 403which does not overlap with the metal oxide film 427 might be removed.In this case, the thickness of the region of the oxide semiconductorfilm 403 which does not overlap with the metal oxide film 427 isreduced.

In the case where the oxide semiconductor film 403 is not processed intoan island shape in the above step, the oxide semiconductor film 403 maybe processed into an island shape simultaneously with the metal oxidefilm 427. By thus patterning the oxide semiconductor film 403 and themetal oxide film 427 simultaneously, the number of photolithographysteps can be reduced. Further, the oxide semiconductor film 403 and themetal oxide film 427 are patterned using the same mask; thus, side edgesin a channel length direction of the oxide semiconductor film 403 arealigned with those of the metal oxide film 407. In this case, the gateinsulating film 402, the oxide semiconductor film 403, and the metaloxide film 427 are preferably formed successively without being exposedto the air.

The metal oxide film 427 is not necessarily processed into an islandshape. For example, openings in which parts of the oxide semiconductorfilm 403 are exposed may be provided so that the oxide semiconductorfilm 403 can be electrically connected to the source electrode 405 a andthe drain electrode 405 b in a later step. With such a structure, thetransistor 320 in FIG. 3B, the transistor 340 in FIG. 4B, the transistor360 in FIG. 4D, and the transistor 380 in FIG. 4F can be formed.

Next, a conductive film for forming the source electrode and the drainelectrode (including a wiring formed in the same layer as the sourceelectrode and the drain electrode) is formed to cover the metal oxidefilm 407 and the oxide semiconductor film 403. As the conductive filmused for the source electrode layer and the drain electrode layer, forexample, a metal film including an element selected from Al, Cr, Cu, Ta,Ti, Mo, and W, a metal nitride film including any of the above elementsas its component (e.g., a titanium nitride film, a molybdenum nitridefilm, or a tungsten nitride film), or the like can be used. Ahigh-melting-point metal film of Ti, Mo, W, or the like or a metalnitride film of any of these elements (a titanium nitride film, amolybdenum nitride film, or a tungsten nitride film) may be stacked onone of or both a bottom side and a top side of a metal film of Al, Cu,or the like. A metal film having a high melting point of Ti, Mo, W, orthe like or a metal nitride film of any of these elements (a titaniumnitride film, a molybdenum nitride film, or a tungsten nitride film) maybe stacked on one of or both a lower side and an upper side of a metalfilm of Al, Cu, or the like. Alternatively, the conductive film to bethe source electrode and the drain electrode may be formed using aconductive metal oxide. As conductive metal oxide, indium oxide (In₂O₃),tin oxide (SnO₂), zinc oxide (ZnO), indium oxide-tin oxide alloy(In₂O₃—SnO₂; abbreviated to ITO), indium oxide-zinc oxide alloy(In₂O₃—ZnO), or any of these metal oxide materials in which silicon orsilicon oxide is contained can be used.

Then, a resist mask is formed over the conductive film in a fourthphotolithography step, and selective etching is performed so that thesource electrode 405 a and the drain electrode 405 b are formed, andthen, the resist mask is removed (see FIG. 5D). When ultraviolet light,KrF laser light, or ArF laser light is used for light exposure forforming the resist mask in a third photolithography step, a distancebetween a lower end of the source electrode 405 a and a lower end of thedrain electrode 405 b that are adjacent to each other over the metaloxide film 407; thus, ultraviolet light, KrF laser light, or ArF laserlight is preferably used in a fourth photolithography step.

In order to reduce the number of photomasks and steps in thephotolithography step, an etching step may be performed with the use ofa resist mask formed using a multi-tone mask which is a light-exposuremask through which light is transmitted so as to have variousintensities. A resist mask formed with the use of a multi-tone mask hasvarious thicknesses and further can be changed in shape by etching;therefore, the resist mask can be used in a plurality of etching stepsfor different patterns. Therefore, a resist mask corresponding to atleast two or more kinds of different patterns can be formed by onemulti-tone mask. Thus, the number of light-exposure masks can be reducedand the number of corresponding photolithography steps can also bereduced, whereby simplification of a manufacturing process can berealized.

The metal oxide film 407 is provided in a region overlapping with achannel formation region of the oxide semiconductor film 403; thus,damage to the channel formation region by etching of the conductive film(e.g., damage due to plasma or etchant in etching) can be prevented.Thus, a semiconductor device including an oxide semiconductor withstable electric characteristics can be provided.

Next, the insulating film 409 covering the source electrode 405 a andthe drain electrode 405 b is preferably formed over and in contact withthe metal oxide film 407 (see FIG. 5E). It is desirable to use amaterial with which a charge trapping center can be formed at theinterface with the metal oxide film 407 when the material is in contactwith the metal oxide film 407, for the insulating film 409. By usingsuch a material for the insulating film 409, charge is trapped at theinterface between the insulating film 409 and the metal oxide film 407,so that it is possible to sufficiently suppress trapping of charge atthe interface between the metal oxide film 407 and the oxidesemiconductor film 403.

The insulating film 409 may be formed using an inorganic film, forexample, a single layer or a stack of any of oxide insulating films suchas a silicon oxide film, a silicon oxynitride film, an aluminum oxidefilm, and an aluminum oxynitride film, and nitride insulating films suchas a silicon nitride film, a silicon nitride oxide film, an aluminumnitride film, and an aluminum nitride oxide film. For example, a siliconoxide film and a silicon nitride film are sequentially formed to bestacked from the metal oxide film 407 side by a sputtering method. Theinsulating film 409 preferably contains a constituent different from theconstituents contained in the oxide semiconductor film 403 and the metaloxide film 407. Note that the insulating film 409 is preferably asilicon oxide film in order that impurities such as hydrogen andmoisture may be removed from the metal oxide film 407 efficiently in astep of heat treatment performed on the oxide semiconductor film 403later. In addition, the energy gap of the insulating film 409 isdesirably larger than that of the metal oxide film 407 because theinsulating film 409 is in contact with the metal oxide film 407.

Note that it is not necessary to limit the material of the insulatingfilm 409 to the above as long as a charge trapping center can be formedat the interface between the insulating film 409 and the metal oxidefilm 407. Further, treatment through which a charge trapping center isformed may be performed on the interface between the insulating film 409and the metal oxide film 407. As such treatment, plasma treatment andtreatment for adding an element (ion implantation or the like) aregiven, for example.

After that, second heat treatment is preferably performed while part ofthe oxide semiconductor film 403 (channel formation region) is incontact with the metal oxide film 407. The second heat treatment isperformed at a temperature higher than or equal to 250° C. and lowerthan or equal to 700° C., preferably higher than or equal to 450° C. andlower than or equal to 600° C. The temperature of the first heattreatment is preferably lower than the strain point of the substrate.

The second heat treatment may be performed in an atmosphere of nitrogen,oxygen, ultra-dry air (air in which a water content is lower than orequal to 20 ppm, preferably lower than or equal to 1 ppm, furtherpreferably lower than or equal to 10 ppb), or a rare gas (argon, helium,or the like). Note that it is preferable that water, hydrogen, or thelike be not contained in the atmosphere of nitrogen, oxygen, ultra-dryair, or a rare gas. It is also preferable that the purity of nitrogen,oxygen, or the rare gas which is introduced into a heat treatmentapparatus be set to higher than or equal to 6N (99.9999%), preferablyhigher than or equal to 7N (99.99999%) (that is, the impurityconcentration is lower than or equal to 1 ppm, preferably lower than orequal to 0.1 ppm).

The second heat treatment is performed while the oxide semiconductorfilm 403 and the metal oxide film 407 are in contact with each other.Thus, oxygen which is one of main constituent materials of the oxidesemiconductor and may be reduced due to the dehydration (ordehydrogenation) treatment can be supplied from the metal oxide film 407containing oxygen to the oxide semiconductor film 403. Accordingly,charge trapping centers in the oxide semiconductor film 403 can bedecreased. Through the above steps, the oxide semiconductor film 403 canbe highly purified to be electrically i-type (intrinsic). In addition,impurities are removed from the metal oxide film 407 at the same time bythis heat treatment, and the metal oxide film 407 can be highlypurified.

Note that in this embodiment, the second heat treatment is performedafter formation of the insulating film 409; however, there is noparticular limitation on the timing of the second heat treatment as longas it is performed after formation of the metal oxide film 407. Forexample, the second heat treatment may be performed after the metaloxide film 407 is formed. Alternatively, when the insulating film 409 isformed by stacking, for example, a silicon oxide film and a siliconnitride film, the second heat treatment may be performed after thesilicon oxide film is formed over the metal oxide film 407 and then, thesilicon nitride film may be formed thereover. Furthermore, the firstheat treatment and the second heat treatment may be successivelyperformed, the first heat treatment may also serve as the second heattreatment, or the second heat treatment may also serve as the first heattreatment.

By performing at least one of the first heat treatment and the secondheat treatment as described above, the oxide semiconductor film 403 canbe purified so as not to contain impurities other than main componentsas little as possible. The number of carriers in the highly-purifiedoxide semiconductor film 403 is very small (close to zero), and thecarrier concentration is less than 1×10¹⁴/cm³, preferably less than1×10¹²/cm³, further preferably less than 1×10¹¹/cm³.

Through the above steps, the transistor 310 is formed (see FIG. 5E). Thetransistor 310 is a transistor including the oxide semiconductor film403 which is highly purified and from which impurities such as hydrogen,moisture, a hydroxyl group, or hydride (also referred to as a hydrogencompound) are removed. Therefore, variation in the electriccharacteristics of the transistor 310 is suppressed and the transistor310 is electrically stable.

In the transistor including the metal oxide film 407, generation of aparasitic channel on the back channel side of the oxide semiconductorfilm 403 can be prevented. By preventing the generation of a parasiticchannel on the back channel side of the oxide semiconductor film 403 inthe transistor 310, variation in the threshold voltage can besuppressed, whereby the reliability of the transistor can be improved.

Note that after the insulating film 409 is formed, the conductive film410 is further formed in a region over the insulating film 409 andoverlapping with a channel formation region of the oxide semiconductorfilm 403, so that the transistor 330 in FIG. 4A and the transistor 340in FIG. 4B can be formed. Note that if the gate electrode 401 and thegate insulating film 402 are not provided, transistors 350 and 360 inFIGS. 4C and 4D can be formed. The conductive film 410 can be formedusing a material and a method which are similar to those of the gateelectrode 401. The conductive film 410 is provided so as to overlap withthe channel formation region of the oxide semiconductor film 403,whereby in a bias-temperature stress test (referred to as a BT test) forexamining the reliability of the transistor 340, the amount of change inthe threshold voltage of the transistor 340 before and after the BT testcan be further reduced. Note that the conductive film 410 may have thesame potential as the gate electrode 401 or have a potential differentfrom that of the gate electrode 401 and may function as a second gateelectrode. The potential of the conductive film 410 may be GND or 0 V,or the conductive film 410 may be in a floating state.

Although not illustrated, a protective insulating film may be furtherformed so as to cover the transistor 310. As the protective insulatingfilm, a silicon nitride film, a silicon nitride oxide film, an aluminumnitride film, or the like can be used.

A planarizing insulating film may be formed over the transistor 310. Theplanarization insulating film can be formed of a heat-resistant organicmaterial such as acrylic, polyimide, benzocyclobutene, polyamide, orepoxy. Other than such organic materials, it is also possible to use alow-dielectric constant material (a low-k material), a siloxane-basedresin, PSG (phosphosilicate glass), BPSG (borophosphosilicate glass), orthe like. Note that the planarizing insulating film may be formed bystacking a plurality of insulating films formed of any of thesematerials.

As described above, in the transistor according to this embodiment, thetop surface portion of the oxide semiconductor film is provided with themetal oxide film containing a constituent similar to that of the oxidesemiconductor film. Thus, the metal oxide film containing a materialcompatible with the oxide semiconductor film is provided in contact withthe oxide semiconductor film, whereby suppressed is trapping of chargeor the like, which is generated due to the operation of a semiconductordevice, at the interface between the oxide semiconductor film and themetal oxide film. Consequently, the oxide semiconductor film can be lessadversely affected by charge, which suppresses fluctuation in thethreshold voltage of the transistor due to trapping of charge at theinterface of the oxide semiconductor film.

Further, an insulating film containing a different constituent from themetal oxide film and the oxide semiconductor film is formed in contactwith a surface of the metal oxide film, which is opposite to the surfacein contact with the oxide semiconductor film. Thus, an insulatorcontaining a material with which a charge trapping center can be formedat the interface is provided in contact with the metal oxide film,whereby the charge can be trapped preferentially at the interfacebetween the metal oxide film and the insulator compared to the interfacebetween the oxide semiconductor film and the metal oxide film.Consequently, the oxide semiconductor film can be less adverselyaffected by charge, which further suppresses fluctuation in thethreshold voltage of the transistor due to trapping of charge at theinterface of the oxide semiconductor film.

The oxide semiconductor film used for the active layer of the transistoris an oxide semiconductor film highly purified to be electrically i-type(intrinsic) by heat treatment in which impurities such as hydrogen,moisture, a hydroxyl group, or a hydride (also referred to as a hydrogencompound) are removed from the oxide semiconductor and oxygen which is amajor constituent of the oxide semiconductor and is reduced concurrentlywith a step of removing impurities is supplied. The transistor includingthe oxide semiconductor film highly purified in such a manner haselectric characteristics which are less likely to change, and thus iselectrically stable.

When charge is trapped at the interface of the oxide semiconductor film,the threshold voltage of the transistor shifts (for example, whenpositive charge is trapped on the back channel side, the thresholdvoltage of the transistor shifts in a negative direction). As one offactors of such charge trapping, the model in which cations (or atomswhich are sources of the cations) travel and are trapped can besupposed, for example. In the transistor including an oxidesemiconductor, such cation sources may be hydrogen atoms. In thedisclosed invention, the highly purified oxide semiconductor is used andis in contact with the stack of the metal oxide film and the insulatingfilm, so that it is possible to suppress even charge trapping caused byhydrogen, which may be caused in the above model. The above model issupposed to be realized when the ionization rate of hydrogen is, forexample, about 10%

As described above, a semiconductor device including an oxidesemiconductor having stable electric characteristics can be provided.Therefore, a semiconductor device with high reliability can be provided.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

EMBODIMENT 2

A semiconductor device (also referred to as a display device) with adisplay function can be manufactured using the transistor an example ofwhich is described in Embodiment 1. Some or all of driver circuitsincluding the transistors can be formed over a substrate where a pixelportion is formed, whereby a system-on-panel can be obtained.

In FIG. 6A, a sealant 4005 is provided to surround a pixel portion 4002provided over a first substrate 4001, and the pixel portion 4002 issealed with the sealant 4005 and the second substrate 4006. In FIG. 6A,a scan line driver circuit 4004 and a signal line driver circuit 4003each are formed using a single crystal semiconductor film or apolycrystalline semiconductor film over a substrate prepared separately,and mounted in a region different from the region surrounded by thesealant 4005 over the first substrate 4001. Various signals andpotentials are supplied to the signal line driver circuit 4003 and thescan line driver circuit 4004 each of which is separately formed, andthe pixel portion 4002, from flexible printed circuits (FPCs) 4018 a and4018 b.

In FIGS. 6B and 6C, the sealant 4005 is provided to surround the pixelportion 4002 and the scan line driver circuit 4004 which are providedover the first substrate 4001. The second substrate 4006 is providedover the pixel portion 4002 and the scan line driver circuit 4004. Thus,the pixel portion 4002 and the scan line driver circuit 4004 are sealedtogether with a display element, by the first substrate 4001, thesealant 4005, and the second substrate 4006. In FIGS. 6B and 6C, thesignal line driver circuit 4003 is formed using a single crystalsemiconductor film or a polycrystalline semiconductor film over asubstrate prepared separately, and mounted in a region different fromthe region surrounded by the sealant 4005 over the first substrate 4001.In FIGS. 6B and 6C, various signals and potentials are supplied to theseparately formed signal line driver circuit 4003, the scan line drivercircuit 4004, and the pixel portion 4002, from an FPC 4018.

Although FIGS. 6B and 6C each show the example in which the signal linedriver circuit 4003 is formed separately and mounted on the firstsubstrate 4001, the present invention is not limited to this structure.The scan line driver circuit may be separately formed and then mounted,or only part of the signal line driver circuit or part of the scan linedriver circuit may be separately formed and then mounted.

Note that a method for connecting a separately formed driver circuit isnot particularly limited, and a chip on glass (COG) method, a wirebonding method, a tape automated bonding (TAB) method, or the like canbe used. FIG. 6A shows an example in which the signal line drivercircuit 4003 and the scan line driver circuit 4004 are mounted by a COGmethod. FIG. 6B shows an example in which the signal line driver circuit4003 is mounted by a COG method. FIG. 6C shows an example in which thesignal line driver circuit 4003 is mounted by a TAB method.

The display device includes in its category a panel in which a displayelement is sealed, and a module in which an IC such as a controller ismounted on the panel.

Note that a display device in this specification means an image displaydevice, a display device, or a light source (including a lightingdevice). The display device also includes the following modules in itscategory: a module to which a connector such as an FPC, a TAB tape, or aTCP is attached; a module having a TAB tape or a TCP at the tip of whicha printed wiring board is provided; and a module in which an integratedcircuit (IC) is directly mounted on a display element by a COG method.

The pixel portion and the scan line driver circuit provided over thefirst substrate include a plurality of transistors and any of thetransistors which are described in Embodiment 1 can be applied.

As the display element provided in the display device, a liquid crystalelement (also referred to as a liquid crystal display element) or alight-emitting element (also referred to as a light-emitting displayelement) can be used. The light-emitting element includes, in itscategory, an element whose luminance is controlled by a current orvoltage, and specifically includes, in its category, an inorganicelectroluminescent (EL) element, an organic EL element, and the like.Furthermore, a display medium whose contrast is changed by an electriceffect, such as electronic ink, can be used.

One embodiment of the semiconductor device will be described withreference to FIG. 7, FIG. 8, and FIG. 9. FIG. 7, FIG. 8, and FIG. 9correspond to cross-sectional views taken along line M-N in FIG. 6B.

As shown in FIG. 7, FIG. 8, and FIG. 9, the semiconductor deviceincludes a connection terminal electrode 4015 and a terminal electrode4016. The connection terminal electrode 4015 and the terminal electrode4016 are electrically connected to a terminal included in the FPC 4018through an anisotropic conductive film 4019.

The connection terminal electrode 4015 is formed of the same conductivefilm as a first electrode layer 4030. The terminal electrode 4016 isformed of the same conductive film as a source electrode and a drainelectrode of transistors 4010 and 4011.

Each of the pixel portion 4002 and the scan line driver circuit 4004provided over the first substrate 4001 includes a plurality oftransistors. In FIG. 7, FIG. 8, and FIG. 9, the transistor 4010 includedin the pixel portion 4002 and the transistor 4011 included in the scanline driver circuit 4004 are shown as an example.

In this embodiment, any of the transistors shown in Embodiment 1 can beapplied to the transistors 4010 and 4011. Variation in the electriccharacteristics of the transistors 4010 and 4011 is suppressed and thetransistors 4010 and 4011 are electrically stable. As described above, asemiconductor device with high reliability as the semiconductor devicesshown in FIG. 7, FIG. 8, and FIG. 9 can be obtained.

The transistor 4010 provided in the pixel portion 4002 is electricallyconnected to the display element to constitute a display panel. Avariety of display elements can be used as the display element as longas display can be performed.

An example of a liquid crystal display device using a liquid crystalelement as a display element is shown in FIG. 7. In FIG. 7, a liquidcrystal element 4013 is a display element including the first electrodelayer 4030, a second electrode layer 4031, and a liquid crystal layer4008. Note that the insulating films 4032 and 4033 serving as alignmentfilms are provided so that the liquid crystal layer 4008 is interposedtherebetween. The second electrode layer 4031 is formed on the secondsubstrate 4006 side. The first electrode layer 4030 and the secondelectrode layer 4031 are stacked with the liquid crystal layer 4008interposed therebetween.

A columnar spacer 4035 is obtained by selective etching of an insulatingfilm and is provided in order to control the thickness (a cell gap) ofthe liquid crystal layer 4008. Alternatively, a spherical spacer may beused.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer dispersed liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like can be used. Such a liquid crystal material exhibits acholesteric phase, a smectic phase, a cubic phase, a chiral nematicphase, an isotropic phase, or the like depending on conditions.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases generated just before a cholesteric phase changes into anisotropic phase while temperature of cholesteric liquid crystal isincreased. Since the blue phase appears only in a narrow temperaturerange, a liquid crystal composition in which several weight percent ormore of a chiral material is mixed is used for the liquid crystal layerin order to improve the temperature range. The liquid crystalcomposition which includes a liquid crystal showing a blue phase and achiral agent has a short response time of 1 msec or less, has opticalisotropy, which makes the alignment process unneeded, and has a smallviewing angle dependence. In addition, since an alignment film does notneed to be provided and rubbing treatment is unnecessary, electrostaticdischarge damage caused by the rubbing treatment can be prevented anddefects and damage of the liquid crystal display device can be reducedin the manufacturing process. Thus, productivity of the liquid crystaldisplay device can be increased.

The specific resistivity of the liquid crystal material is 1×10⁹ Ω·cm ormore, preferably 1×10¹¹ Ω·cm or more, further preferably 1×10¹² Ω·cm ormore. Note that the specific resistivity in this specification ismeasured at 20° C.

The size of a storage capacitor provided in the liquid crystal displaydevice is set considering the leakage current of the transistor providedin the pixel portion or the like so that charge can be held for apredetermined period. Since the transistor including a high-purity oxidesemiconductor film is used, a storage capacitor having capacitance whichis ⅓ or less, preferably ⅕ or less with respect to a liquid crystalcapacitance of each pixel is sufficient to be provided.

In the transistor used in this embodiment, which uses a highly-purifiedoxide semiconductor film, the current in an off state (the off-statecurrent) can be made small. Therefore, an electrical signal such as animage signal can be held for a long period, and a writing interval canbe set long when the power is on. Consequently, frequency of refreshoperation can be reduced, which leads to an effect of suppressing powerconsumption.

The field-effect mobility of the transistor including a highly-purifiedoxide semiconductor film used in this embodiment can be relatively high,whereby high-speed operation is possible. Thus, by using the transistorin a pixel portion of the liquid crystal display device, a high-qualityimage can be provided. In addition, since the transistors can beseparately provided in a driver circuit portion and a pixel portion overone substrate, the number of components of the liquid crystal displaydevice can be reduced.

For the liquid crystal display device, a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, anaxially symmetric aligned micro-cell (ASM) mode, an optical compensatedbirefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, anantiferroelectric liquid crystal (AFLC) mode, and the like can be used.

A normally black liquid crystal display device such as a transmissiveliquid crystal display device utilizing a vertical alignment (VA) modeis preferable. The vertical alignment mode is one of methods ofcontrolling alignment of liquid crystal molecules of a liquid crystaldisplay panel. The vertical alignment mode is a mode in which liquidcrystal molecules are aligned vertically to a panel surface when voltageis not applied. Some examples are given as the vertical alignment mode.For example, a multi-domain vertical alignment (MVA) mode, a patternedvertical alignment (PVA) mode, an Advanced Super View (ASV) mode, andthe like can be used. Moreover, it is possible to use a method calleddomain multiplication or multi-domain design, in which a pixel isdivided into some regions (subpixels) and molecules are aligned indifferent directions in their respective regions.

In the display device, a black matrix (a light-blocking layer), anoptical member (an optical substrate) such as a polarizing member, aretardation member, or an anti-reflection member, and the like areprovided as appropriate. For example, circular polarization may beemployed by using a polarizing substrate and a retardation substrate. Inaddition, a backlight, a side light, or the like may be used as a lightsource.

In addition, with the use of a plurality of light-emitting diodes (LEDs)as a backlight, a time-division display method (a field-sequentialdriving method) can be employed. With the field-sequential drivingmethod, color display can be performed without using a color filter.

As a display method in the pixel portion, a progressive method, aninterlace method, or the like can be employed. Color elements controlledin a pixel at the time of color display are not limited to three colors:R, G, and B (R, G, and B correspond to red, green, and bluerespectively). For example, R, G, B, and W (W corresponds to white), orR, G, B, and one or more of yellow, cyan, magenta, and the like can beused. The sizes of display regions may be different between respectivedots of color elements. Note that the present invention is not limitedto the application to a display device for color display but can also beapplied to a display device for monochrome display.

Alternatively, as the display element included in the display device, alight-emitting element utilizing electroluminescence can be used.Light-emitting elements utilizing electroluminescence are classifiedaccording to whether a light-emitting material is an organic compound oran inorganic compound. In general, the former is referred to as anorganic EL element, and the latter is referred to as an inorganic ELelement.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and current flows. The carriers (electrons and holes) are recombined,and thus the light-emitting organic compound is excited. Thelight-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Owing to such a mechanism, such alight-emitting element is referred to as a current-excitationlight-emitting element.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that an example ofan organic EL element as a light-emitting element is described here.

In order to extract light emitted from the light-emitting element, it isacceptable as long as at least one of a pair of electrodes istransparent. Then a transistor and a light-emitting element are formedover a substrate. The light-emitting element can have any of thefollowing structure: a top emission structure in which light isextracted through the surface opposite to the substrate; a bottomemission structure in which light is extracted through the surface onthe substrate side; or a dual emission structure in which light isextracted through the surface opposite to the substrate and the surfaceon the substrate side.

An example of a light-emitting device using a light-emitting element asa display element is shown in FIG. 8. A light-emitting element 4513which is a display element is electrically connected to the transistor4010 provided in the pixel portion 4002. The light-emitting element 4513has a stacked-layer structure of the first electrode layer 4030, anelectroluminescent layer 4511, and the second electrode layer 4031 butis not limited to this structure. The structure of the light-emittingelement 4513 can be changed as appropriate depending on a direction inwhich light is extracted from the light-emitting element 4513, or thelike.

A partition wall 4510 can be formed using an organic insulating materialor an inorganic insulating material. It is particularly preferable thatthe partition wall 4510 be formed using a photosensitive resin materialto have an opening portion over the first electrode layer 4030 so that asidewall of the opening portion is formed as a tilted surface withcontinuous curvature.

The electroluminescent layer 4511 may be formed with either a singlelayer or a stacked layer of a plurality of layers.

A protective film may be formed over the second electrode layer 4031 andthe partition wall 4510 in order to prevent entry of oxygen, hydrogen,moisture, carbon dioxide, or the like into the light-emitting element4513. As the protective film, a silicon nitride film, a silicon nitrideoxide film, a diamond like carbon (DLC) film, or the like can be formed.In a space sealed with the first substrate 4001, the second substrate4006, and the sealant 4005, a filler 4514 is provided and tightlysealed. It is preferable that the light-emitting element be packaged(sealed) with a cover material with high air-tightness and littledegasification or a protective film (such as a laminate film or anultraviolet curable resin film) so that the light-emitting element isnot exposed to the outside air, in this manner.

As the filler 4514, an ultraviolet curable resin or a thermosettingresin can be used as well as an inert gas such as nitrogen or argon, andpolyvinyl chloride (PVC), acrylic, polyimide, an epoxy resin, a siliconeresin, polyvinyl butyral (PVB), ethylene vinyl acetate (EVA), or thelike can be used. For example, nitrogen is used for the filler.

If needed, an optical film, such as a polarizing plate, a circularlypolarizing plate (including an elliptically polarizing plate), aretardation plate (a quarter-wave plate or a half-wave plate), or acolor filter, may be provided as appropriate on a light-emitting surfaceof the light-emitting element. Further, the polarizing plate or thecircularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on the surface so as to reducethe glare can be performed.

An electronic paper in which electronic ink is driven can be provided asthe display device. The electronic paper is also called anelectrophoretic display device (electrophoretic display) and hasadvantages in that it has the same level of readability as regularpaper, it has less power consumption than other display devices, and itcan be set to have a thin and light form.

An electrophoretic display device can have various modes. Anelectrophoretic display device contains a plurality of microcapsulesdispersed in a solvent or a solute, each microcapsule containing firstparticles which are positively charged and second particles which arenegatively charged. By applying an electric field to the microcapsules,the particles in the microcapsules move in opposite directions to eachother and only the color of the particles gathering on one side isdisplayed. Note that the first particles and the second particles eachcontain pigment and do not move without an electric field. Moreover, thefirst particles and the second particles have different colors (whichmay be colorless).

Thus, an electrophoretic display device is a display device thatutilizes a so-called dielectrophoretic effect by which a substancehaving a high dielectric constant moves to a high-electric field region.

A solution in which the above microcapsules are dispersed in a solventis referred to as electronic ink. This electronic ink can be printed ona surface of glass, plastic, cloth, paper, or the like. Furthermore, byusing a color filter or particles that have a pigment, color display canalso be achieved.

Note that the first particles and the second particles in themicrocapsules may each be formed of a single material selected from aconductive material, an insulating material, a semiconductor material, amagnetic material, a liquid crystal material, a ferroelectric material,an electroluminescent material, an electrochromic material, and amagnetophoretic material, or formed of a composite material of any ofthese.

As an electronic paper, a display device using a twisting ball displaymethod can be used. The twisting ball display method refers to a methodin which spherical particles each colored in white and black arearranged between a first electrode layer and a second electrode layerwhich are electrode layers used for a display element, and a potentialdifference is generated between the first electrode layer and the secondelectrode layer to control orientation of the spherical particles, sothat display is performed.

FIG. 9 shows an active matrix electronic paper as one embodiment of asemiconductor device. The electronic paper in FIG. 9 is an example of adisplay device using a twisting ball display method.

Between the first electrode layer 4030 connected to the transistor 4010and the second electrode layer 4031 provided on the second substrate4006, spherical particles 4613 each of which includes a black region4615 a, a white region 4615 b, and a cavity 4612 around the regionswhich is filled with liquid, are provided. A space around the sphericalparticles 4613 is filled with a filler 4614 such as a resin. The secondelectrode layer 4031 corresponds to a common electrode (counterelectrode). The second electrode layer 4031 is electrically connected toa common potential line.

Note that in FIG. 7, FIG. 8, and FIG. 9, a flexible substrate as well asa glass substrate can be used as the first substrate 4001 and the secondsubstrate 4006. For example, a plastic substrate havinglight-transmitting properties can be used. For plastic, afiberglass-reinforced plastics (FRP) plate, a polyvinyl fluoride (PVF)film, a polyester film, or an acrylic resin film can be used. A sheetwith a structure in which an aluminum foil is sandwiched between PVFfilms or polyester films can also be used.

The insulating layer 4021 can be formed using an organic insulatingmaterial or an inorganic insulating material. Note that an organicinsulating material having heat resistance, such as an acrylic resin, apolyimide, a benzocyclobutene-based resin, a polyamide, or an epoxyresin is preferably used as a planarizing insulating film. Other thansuch organic insulating materials, it is possible to use alow-dielectric constant material (a low-k material), a siloxane-basedresin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), orthe like. The insulating layer may be formed by stacking a plurality ofinsulating films formed of these materials.

There is no particular limitation on the method for forming theinsulating layer 4021, and the insulating layer 4021 can be formed,depending on a material thereof, by a sputtering method, a spin coatingmethod, a dipping method, a spray coating method, a droplet dischargingmethod (e.g., an ink jet method, a screen printing method, or an offsetprinting method), a roll coating method, a curtain coating method, aknife coating method, or the like.

The display device performs display by transmitting light from a lightsource or a display element. Thus, the substrates and the thin filmssuch as insulating films and conductive films provided in the pixelportion where light is transmitted have light-transmitting propertieswith respect to light in the visible-light wavelength range.

The first electrode layer and the second electrode layer (each of whichmay be called a pixel electrode layer, a common electrode layer, acounter electrode layer, or the like) for applying voltage to thedisplay element may have light-transmitting properties orlight-reflecting properties, which depends on the direction in whichlight is extracted, the position where the electrode layer is provided,and the pattern structure of the electrode layer.

A light-transmitting conductive material such as indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium tin oxide (hereinafter referred to as ITO), indium zincoxide, or indium tin oxide to which silicon oxide is added, can be usedfor the first electrode layer 4030 and the second electrode layer 4031.

The first electrode layer 4030 and the second electrode layer 4031 canbe formed using one kind or plural kinds selected from metal such astungsten (W), molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium(V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel(Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper (Cu), orsilver (Ag); an alloy thereof; and a nitride thereof.

A conductive composition containing a conductive high molecule (alsoreferred to as a conductive polymer) can be used for the first electrodelayer 4030 and the second electrode layer 4031. As the conductive highmolecule, a so-called π-electron conjugated conductive polymer can beused. For example, polyaniline or a derivative thereof, polypyrrole or aderivative thereof, polythiophene or a derivative thereof, and acopolymer of two or more of aniline, pyrrole, and thiophene or aderivative thereof can be given.

Since the transistor is easily broken due to static electricity or thelike, a protective circuit for protecting the driver circuit ispreferably provided. The protective circuit is preferably formed using anonlinear element.

As described above, by using any of the transistors shown in Embodiment1, a semiconductor device having a high reliability can be provided.Note that the transistors described in Embodiment 1 can be applied tonot only semiconductor devices having the display functions describedabove but also semiconductor devices having a variety of functions, suchas a power device which is mounted on a power supply circuit, asemiconductor integrated circuit such as an LSI, and a semiconductordevice having an image sensor function of reading information of anobject.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

EMBODIMENT 3

A semiconductor device disclosed in this specification can be applied toa variety of electronic appliances (including game machines). Examplesof electronic appliances are a television set (also referred to as atelevision or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a portable game machine, aportable information terminal, an audio reproducing device, alarge-sized game machine such as a pachinko machine, and the like.Examples of electronic appliances each including the semiconductordevice described in the above embodiment will be described.

FIG. 10A illustrates a laptop personal computer, which includes a mainbody 3001, a housing 3002, a display portion 3003, a keyboard 3004, andthe like. By applying the semiconductor device described in Embodiment 1or 2, the laptop personal computer can have high reliability.

FIG. 10B is a portable information terminal (PDA) which includes adisplay portion 3023, an external interface 3025, an operation button3024, and the like in a main body 3021. A stylus 3022 is included as anaccessory for operation. By applying the semiconductor device describedin Embodiment 1 or 2, the portable information terminal (PDA) can havehigher reliability.

FIG. 10C illustrates an example of an electronic book reader. Forexample, an electronic book reader 2700 includes two housings, a housing2701 and a housing. 2703. The housing 2701 and the housing 2703 arecombined with a hinge 2711 so that the electronic book reader 2700 canbe opened and closed with the hinge 2711 as an axis. With such astructure, the electronic book reader 2700 can operate like a paperbook.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. The display portion2705 and the display portion 2707 may display one image or differentimages. When the display portion 2705 and the display portion 2707display different images, for example, text can be displayed on adisplay portion on the right side (the display portion 2705 in FIG. 10C)and graphics can be displayed on a display portion on the left side (thedisplay portion 2707 in FIG. 10C). By applying the semiconductor devicedescribed in Embodiment 1 or 2, the electronic book reader 2700 can havehigh reliability.

FIG. 10C illustrates an example in which the housing 2701 is providedwith an operation portion and the like. For example, the housing 2701 isprovided with a power switch 2721, operation keys 2723, a speaker 2725,and the like. With the operation key 2723, pages can be turned. Notethat a keyboard, a pointing device, or the like may also be provided onthe surface of the housing, on which the display portion is provided.Furthermore, an external connection terminal (an earphone terminal, aUSB terminal, or the like), a recording medium insertion portion, andthe like may be provided on the back surface or the side surface of thehousing. Moreover, the electronic book reader 2700 may have a functionof an electronic dictionary.

The electronic book reader 2700 may have a configuration capable ofwirelessly transmitting and receiving data. Through wirelesscommunication, desired book data or the like can be purchased anddownloaded from an electronic book server.

FIG. 10D illustrates a mobile phone, which includes two housings, ahousing 2800 and a housing 2801. The housing 2801 includes a displaypanel 2802, a speaker 2803, a microphone 2804, a pointing device 2806, acamera lens 2807, an external connection terminal 2808, and the like. Inaddition, the housing 2800 includes a solar cell 2810 having a functionof charge of the mobile phone, an external memory slot 2811, and thelike. Further, an antenna is incorporated in the housing 2801. Byapplying the semiconductor device described in Embodiment 1 or 2, themobile phone can have high reliability.

Further, the display panel 2802 is provided with a touch panel. Aplurality of operation keys 2805 which are displayed as images isillustrated by dashed lines in FIG. 10D. Note that a boosting circuit bywhich a voltage output from the solar cell 2810 is increased to besufficiently high for each circuit is also included.

In the display panel 2802, the display orientation can be appropriatelychanged depending on a usage pattern. Further, the display device isprovided with the camera lens 2807 on the same surface as the displaypanel 2802, and thus it can be used as a video phone. The speaker 2803and the microphone 2804 can be used for videophone calls, recording andplaying sound, and the like as well as voice calls. Moreover, thehousings 2800 and 2801 in a state where they are opened as illustratedin FIG. 10D can be slid so that one overlaps the other; therefore, thesize of the mobile phone can be reduced, which makes the mobile phonesuitable for being carried.

The external connection terminal 2808 can be connected to an AC adapterand various types of cables such as a USB cable, and charging and datacommunication with a personal computer are possible. Moreover, a largeramount of data can be saved and moved by inserting a recording medium tothe external memory slot 2811.

Further, in addition to the above functions, an infrared communicationfunction, a television reception function, or the like may be provided.

FIG. 10E illustrates a digital video camera which includes a main body3051, a display portion A 3057, an eyepiece 3053, an operation switch3054, a display portion B 3055, a battery 3056, and the like. Byapplying the semiconductor device described in Embodiment 1 or 2, thedigital video camera can have high reliability.

FIG. 10F illustrates an example of a television set. In a television set9600, a display portion 9603 is incorporated in a housing 9601. Thedisplay portion 9603 can display images. Here, the housing 9601 issupported by a stand 9605. By applying the semiconductor devicedescribed in Embodiment 1 or 2, the television set can have highreliability.

The television set 9600 can be operated by an operation switch of thehousing 9601 or a separate remote controller. Further, the remotecontroller may be provided with a display portion for displaying dataoutput from the remote controller.

Note that the television set 9600 is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Furthermore, when the display device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

This application is based on Japanese Patent Application serial No.2010-090539 filed with the Japan Patent Office on Apr. 9, 2010, theentire contents of which are hereby incorporated by reference.

The invention claimed is:
 1. A method for forming a semiconductor devicecomprising the steps of: forming a gate electrode; forming a gateinsulating film over the gate electrode; forming an oxide semiconductorfilm comprising a channel formation region over the gate insulatingfilm; forming a metal oxide film over and in contact with the oxidesemiconductor film; performing a heat treatment after forming the metaloxide film; forming a source electrode and a drain electrode over themetal oxide film, the source electrode and the drain electrode being incontact with the oxide semiconductor film; and forming an electrode overthe metal oxide film and overlapping with the gate electrode, whereinthe oxide semiconductor film contains one or more metal elements,wherein the metal oxide film contains at least one of the metal elementscontained in the oxide semiconductor film, and wherein a side surface ofthe oxide semiconductor film is in contact with the metal oxide film. 2.The method for forming a semiconductor device according to claim 1,further comprising the step of: performing a dehydration treatment or adehydrogenation treatment on the oxide semiconductor film, wherein thedehydration treatment or the dehydrogenation treatment is performed at atemperature of 250° C. to 700° C.
 3. The method for forming asemiconductor device according to claim 1, wherein oxygen is supplied tothe oxide semiconductor film from the metal oxide film in the step ofthe heat treatment, and wherein the channel formation region comprisesan intrinsic oxide semiconductor after the heat treatment.
 4. The methodfor forming a semiconductor device according to claim 1, wherein each ofthe oxide semiconductor film and the metal oxide film is formed using asputtering method.
 5. The method for forming a semiconductor deviceaccording to claim 1, wherein the metal oxide film has a larger band gapthan the oxide semiconductor film.
 6. The method for forming asemiconductor device according to claim 1, wherein the oxidesemiconductor film contains indium, gallium and zinc, and wherein themetal oxide film contains at least gallium.
 7. A method for forming asemiconductor device comprising the steps of: forming an oxidesemiconductor film comprising a channel formation region; forming ametal oxide film over and in contact with the oxide semiconductor film;performing a heat treatment after forming the metal oxide film; forminga source electrode and a drain electrode over the metal oxide film, thesource electrode and the drain electrode being in contact with the oxidesemiconductor film; and forming an electrode over the metal oxide film,wherein the electrode overlaps with the channel formation region withthe metal oxide film interposed therebetween, wherein the oxidesemiconductor film contains one or more metal elements, wherein themetal oxide film contains at least one of the metal elements containedin the oxide semiconductor film, and wherein a side surface of the oxidesemiconductor film is in contact with the metal oxide film.
 8. Themethod for forming a semiconductor device according to claim 7, furthercomprising the step of: performing a dehydration treatment or adehydrogenation treatment on the oxide semiconductor film, wherein thedehydration treatment or the dehydrogenation treatment is performed at atemperature of 250° C. to 700° C.
 9. The method for forming asemiconductor device according to claim 7, wherein oxygen is supplied tothe oxide semiconductor film from the metal oxide film in the step ofthe heat treatment, and wherein the channel formation region comprisesan intrinsic oxide semiconductor after the heat treatment.
 10. Themethod for forming a semiconductor device according to claim 7, whereineach of the oxide semiconductor film and the metal oxide film is formedusing a sputtering method.
 11. The method for forming a semiconductordevice according to claim 7, wherein the metal oxide film has a largerband gap than the oxide semiconductor film.
 12. The method for forming asemiconductor device according to claim 7, wherein the oxidesemiconductor film contains indium, gallium and zinc, and wherein themetal oxide film contains at least gallium.
 13. A method for forming asemiconductor device comprising the steps of: forming a gate electrode;forming a gate insulating film over the gate electrode; forming a firstmetal oxide film over the gate insulating film; forming an oxidesemiconductor film comprising a channel formation region over the firstmetal oxide film; forming a second metal oxide film over and in contactwith the oxide semiconductor film; performing a heat treatment afterforming the second metal oxide film; and forming a source electrode anda drain electrode over the second metal oxide film, the source electrodeand the drain electrode being in contact with the oxide semiconductorfilm, wherein the oxide semiconductor film contains one or more metalelements, wherein each of the first metal oxide film and the secondmetal oxide film contains at least one of the metal elements containedin the oxide semiconductor film, and wherein a side surface of the oxidesemiconductor film is in contact with the second metal oxide film. 14.The method for forming a semiconductor device according to claim 13,further comprising the step of: performing a dehydration treatment or adehydrogenation treatment on the oxide semiconductor film, wherein thedehydration treatment or the dehydrogenation treatment is performed at atemperature of 250° C. to 700° C.
 15. The method for forming asemiconductor device according to claim 13, wherein oxygen is suppliedto the oxide semiconductor film from the first metal oxide film and thesecond metal oxide film in the step of the heat treatment, and whereinthe channel formation region comprises an intrinsic oxide semiconductorafter the heat treatment.
 16. The method for forming a semiconductordevice according to claim 13, wherein each of the oxide semiconductorfilm, the first metal oxide film and the second metal oxide film isformed using a sputtering method.
 17. The method for forming asemiconductor device according to claim 13, wherein each of the firstmetal oxide film and the second metal oxide film has a larger band gapthan the oxide semiconductor film.
 18. The method for forming asemiconductor device according to claim 13, wherein the oxidesemiconductor film contains indium, gallium and zinc, and wherein eachof the first metal oxide film and the second metal oxide film containsat least gallium.
 19. The method for forming a semiconductor deviceaccording to claim 13, further comprising the steps of: forming anelectrode over the second metal oxide film and overlapping with the gateelectrode.